Certain pests are very destructive to agricultural commodities; thus quarantine restrictions are imposed to ensure that these pests are not disseminated by export of agricultural commodities which may harbor these pests to areas where the pests do not occur. Illustrative of such quarantine pests are tephritid fruit flies (Diptera: Tephritidae) such as the Mediterranean fruit fly, Ceratitis capitata (Wiedemann); the oriental fruit fly, Dacus dorsalis Hendel, and the melon fly, Dacus cucurbitae Coquillet. Fruit flies are among the most destructive insect pests of citrus, deciduous fruits, and vegetables. Tephritid fruit flies are present in Hawaii and have been shown to spread as larvae or eggs in fruits or vegetables shipped from infested areas. Agricultural commodities such as papayas, mangos, avocados, citrus, cucumbers, and bell peppers produced in Hawaii that may be infested with fruit flies cannot be shipped to the mainland U.S. or Japanese markets without quarantine treatment to ensure that the fruits or vegetables are free of fruit flies.
Prior to September, 1984, the standard treatment for papayas was fumigation with ethylene dibromide (EDB). Subsequent to the cancellation by the U.S. Environmental Protection Agency of the use of EDB as a food fumigant, the quarantine treatment that was adopted consisted of selecting papaya fruit of no more than quarter-ripeness as defined by color standard values measured with a calorimeter and then immersing the fruit in water at 42 degree(s) C. for 30 minutes followed immediately by a second immersion in water at 49 degree(s) C. for 20 minutes (Couey and Hayes, Journal of Economic Entomology, 79:1307-1314 (1986)). The hot-water immersions are used to destroy the fruit fly eggs and control postharvest decay. However, the complete treatment is limited because of the ripeness selection and because the water immersions are not sufficiently hot to kill fruit fly larvae inside the fruit. This treatment was deregistered as an approved method of quarantine treatment for papayas in 1991.
Another quarantine procedure, called the "vapor heat treatment", uses high-temperature water-saturated vapor to raise the pulp temperature of papaya to 44.4 degrees C. over a 6 to 8 hour period. The fruits are held at the temperature for 8.75 hours, then cooled immediately after the heating phase to below ambient temperature (APHIS, Plant Protection and Quarantine Treatment Manual, Section III, Part 9, Treatment Procedure, 1985). Although this procedure is effective against all fruit fly life stages, the treatment is time consuming and expensive. Also, scalding damage to the fruit may occur. A modified version of the vapor heat treatment, the "quick run-up treatment", requires heating papayas with saturated water vapor until the fruit center temperature reaches 47.2 degree(s) C., then immediately cooling the fruits (APHIS, CFR Amendment No. 85-19, Part 318--Hawaiian and Territorial Quarantine Notices). Although this method takes less time, elaborate facilities are still needed and some fruits may be damaged by vapor heat.
Another method of treating fruit for these infestations comprises heating a fruit or vegetable commodity in hot air under controlled conditions of relative humidity until the temperature of the commodity exceeds the thermal death point temperature of the pest but does not adversely affect the quality of the commodity. The commodity is held at this temperature until the pest is killed. Then, the commodity is cooled. This disinfestation treatment provides a replacement method to the prior art methods that use hot water or saturated water vapor and which have the disadvantages outlined above.
Using the hot air method, fruit and vegetable commodities subject to infestation by quarantine pests are disinfested so as to meet quarantine restrictions without adversely affecting the quality of the commodity. The method involves heating the commodity in hot air having a relative humidity of 30-80 percent until the temperature of the commodity exceeds the thermal death point temperature of the target pest, but is not so high as to adversely affect commodity quality. The temperature is held at this temperature until the pest is killed. The method is effective against all life stages of quarantine pests and is suitable for large-scale commercial disinfestation of commodities for movement through marketing channels, for example, for disinfestation of papayas of tephritid fruit flies such as the Mediterranean fruit fly, the melon fly and the oriental fruit fly.
Under controlled conditions this method does not adversely affect fruit and vegetable quality such as odor, taste, appearance, ripening, texture, shelf-life or other marketable traits of the commodity. Major agricultural commodities which can be treated by this method include, but are not limited to, fruits and vegetables such as papaya, mango, starfruit (carambola), atemoya, lychee, eggplant, green peppers, sweet peppers, hot peppers, cucumbers, avocado and squash.
Disinfestation chambers have been built which employ the different methods described above for killing various pests in different fruits and vegetables. One common chamber in use currently includes a fan, typically centrifugal, housed in its own chamber, separate from the commodity chamber. Such a chamber was taught by Tsuji et al. in U.S. Pat. No. 4,676,152. In the Tsuji chamber air is drawn in, heated by a heat exchanger, and then blown into the commodity chamber. Uniform airflow in such a chamber has proven difficult to maintain. The airflow can be reversed by the use of flow dampers. However, such dampers serve to further disrupt the airflow. The external positioning of the fan relative to the chamber creates heat and air flow losses, as well as temperature variations at different locations within the chamber.
Another known method employs horizontal airflow through the chamber. In this type of chamber the fans or ducts are mounted on the side of the chamber and blow the air across the chamber horizontally. Horizontal airflow is not conducive to successfully treating commodities stored in bins.